The present invention relates to a reflector for a vehicular headlight which uses a discharge lamp as a light source.
Recent trends of automobile design have prompted efforts to develop new types of headlights. That is, with a streamlined body shape to satisfy the requirements of high speed and high fuel efficiency, the front faces of automobiles tend to incline toward the horizontal plane, which necessitates outer lenses of headlights to incline similarly. As a result, the effective height of headlights tends to decrease. As another trend, small metal halide lamps have recently attacted much attention as light sources for such headlights.
FIG. 15 schematically shows a spatial relationship between a metal halide lamp a and a paraboloid-of-revolution reflector b. The metal halide lamp a is disposed so that the central axis of its glass tube c coincides with the optical axis L--L of the reflector a. An arc is generated between electrodes arranged in a sphere d (discharge space) located at the center of the glass tube c.
However, conventionally the shapes of reflector surfaces have been designed with an assumption that a coil-like filament is used; that is, they have been designed with no fundamental consideration of the structure of metal halide lamps. Therefore, the combination of such a reflector and a metal halide lamp is associated with a problem that glare appears conspicuously in a light distribution pattern.
FIG. 16 schematically shows upper edge portions f of images projected onto a front screen by reflecting sectors e (shown hatched in FIG. 15 for clarity) that occupy right and left areas of the reflecting surface of the reflector b of FIG. 15. The reflecting sectors e have predetermined central angles when viewed from the front side.
In FIG. 16, H--H and V--V denote horizontal and vertical lines, respectively. The images f are located under the horizontal line H--H with the vertical line V--V in between, and shaped like a bow which corresponds to the electric arc of the metal halide lamp a.
Glare appears conspicuously in areas g (hatched in FIG. 16) located over the images f and occupying both sides of the horizontal line H--H. The glare is caused by light emission by metal iodide substances accumulated in the sphere d of the metal halide lamp a.
FIG. 17 is an enlarged view of the sphere d of the metal halide lamp a. Electrode rods i are penetrated through pinch seal portions h that are connected to the sphere d on both sides. An arc j is generated between tip portions of the electrode rods i that are projected into the sphere d. The arc j has the maximum brightness at positions p, p that are close to the electrode rods i.
As shown in FIG. 17, there is an accumulation k of metal iodide substances on the bottom of the sphere d. The accumulation k is a cause of secondary light generated in response to the light of the arc j, and the glare in the light-distribution pattern is caused by the light from the accumulation k and a region hatched in FIG. 17.
Since the images f contribute to formation of the cutline (cutoff line) and the maximum contrast portion of the light distribution pattern, the shape of the reflecting sectors e is important for the reduction of the glare.